I have the ILE Porteur Rackbag on a Soma porteur rack and love it.
It's waterproof, expands out huge and has extra long straps integrated in the lid so you don't have to carry separate bungees for big stuff. It clips on and off with a couple of side-release buckles underneath so pretty quick. The opening on top is just a big chunk of velcro so easy to open, dip in, and re-close while riding. I've had it for a couple of years and so far it seems bombproof. It comes with a shoulder strap so can be easily taken into the supermarket or wherever.
Downsides: it's not the lightest thing in the world, and it's not cheap.
I have the ILE Porteur Rackbag on a Soma porteur rack and love it.
It's waterproof, expands out huge and has extra long straps integrated in the lid so you don't have to carry separate bungees for big stuff. It clips on and off with a couple of side-release buckles underneath so pretty quick. The opening on top is just a big chunk of velcro so easy to open, dip in, and re-close while riding. I've had it for a couple of years and so far it seems bombproof. It comes with a shoulder strap so can be easily taken into the supermarket or wherever.
Downsides: it's not the lightest thing in the world, and it's not cheap.
Always Riding or RideAuburn carry it.
https://ilequipment.com/products/velo-orange-pass-stow-rackbag
I managed to get a barbecue plus extra parcels in it a while ago:
1 Attachment